1. Field of the Invention
The present invention relates to light-emitting display devices such as electroluminescent devices and particularly relates to a light-emitting display device having barriers that delimit a display region.
2. Description of the Related Art
Application of flat panel display devices such as liquid crystal devices and electroluminescent (EL) devices is expanding with spread of portable terminals and the like.
EL devices and particularly organic EL devices of a self-luminous type can be driven at a relatively low voltage and can display colors. Such devices are now applied to display devices of mobile phones in practice. Large-sized EL devices are currently under development and are expected to have various applications as displays for personal computers and television sets.
FIG. 1 is a cross-sectional view of a typical organic EL device. The organic EL device has a substrate 1 of glass or the like, an anode 2 of a transparent material such as indium tin oxide (ITO), a hole transport layer 3, an EL layer 4, and a triple-layer cathode 5 of lithium fluoride, calcium, and aluminum. The organic EL device is driven by a direct current. When a DC current is applied between the anode 2 and the cathode 5, holes are injected from the hole transport layer 3 into the EL layer 4 and are combined with electrons injected from the cathode 5 to excite a host material contained in the EL layer 4 or the emission center. As a result, the EL device emits light.
In general, moisture deteriorates materials for the EL layer 4. Thus, wet patterning processes such as etching are not suitable for the formation of the EL layer 4 and overlying layers. Thus, the EL layer 4 and the cathode 5 are generally formed by printing or mask deposition. Incidentally, a color display requires a plurality of light-emitting layers. When these light-emitting layers are formed by printing or mask deposition, a color region readily bleeds toward adjoining color regions during the formation of the color region, resulting in undesirable color mixing. For example, in a full-color display device having picture units, each picture unit including a red pixel 4R, a green pixel 4G, and a blue pixel 4B, as shown in FIG. 2, barriers 6 are formed between these pixels 4R, 4G, and 4B to prevent bleeding of each color material. In the present invention, a minimum unit for controlling emission of the EL layers is referred to as “pixel,” whereas a minimum combination of different colors is referred to as “picture unit.” That is, one picture unit includes three R, G, and B pixels for a three-primary color display or one pixel for a monochrome display. This organic EL device including the barriers 6 is formed by steps shown in FIGS. 3A to 3D. Referring to FIG. 3A, ITO is deposited on a glass substrate 1 and is etched into a predetermined pattern to form an anode 2. Referring to FIG. 3B, a resin material 7 such as a photoresist is applied onto the entire surface of the substrate 1 with the anode 2. The resin material 7 is dried, is exposed with a mask, is developed, and is fired to form barriers 6 as shown in FIG. 3C. Referring to FIG. 3D, hole transport layers 3, EL layers 4, and cathodes 5 are formed between the barriers 6 on the anode 2 by mask deposition or the like. In FIG. 3D, a hole transport layer 3, an EL layer 4, and a cathode 5 are referred to as an “EL structure 8.”
For improving the brightness and efficiency of display, the relationship on the refractive indices between the substrate and air is important. As shown by an optical path b in FIG. 4, when light is incident on the substrate 1 from the EL structure 8 at an angle exceeding a critical angle that is determined by the ratio of the refractive index of the air to that of the substrate, the light is completely reflected at a surface (at a viewer side) in contact with the air. In another optical path c, light is repeatedly reflected in the EL structure 8. Thus, the emergence efficiency of the light from the device to the exterior is at most 15% to 20%.
For solving this problem, a structure shown in FIG. 5 is disclosed in U.S. Pat. No. 4,774,435. A surface, in contact with an EL structure, of a substrate 1 has irregularities 9 with a roughness that is larger than the wavelength of light to cause light scattering on the irregular surface that improves the emergence efficiency. These surface irregularities are formed by etching the substrate surface. An anode, a hole transport layer, an EL layer, and a cathode are formed on the irregular surface in that order.
Unfortunately, a combination of the barriers 6 shown in FIG. 2 and the irregularities 9 shown in U.S. Pat. No. 4,774,435 requires two processing steps, i.e., the formation of the irregularities 9 and the formation of the barriers 6, resulting in increased process cost.
In order to improve the brightness of each pixel of the EL device and to control the pixel precisely, the EL device is preferably of an active matrix type having a switching element such as a thin film transistor (TFT) for every pixel. For providing switching elements having uniform characteristics, the underlying surface on with the switching elements are formed must be flat. Thus, a mask of a resist or the like must be formed before etching to keep flat regions on the surface, resulting in increased process cost.